In recent years, as the demand for higher precision in the field of precision processing technology has been mounting, it has been becoming important to warrant dimensional stability even for the materials to be used for the members forming precision processing machines. The materials for such members, therefore, have been becoming to require higher degrees of low thermal expansion property than ever attainable. They further require high specific rigidity for the purpose of enabling these members to attain reduction in weight and addition to resonance frequency. For such uses as dictate high cleanliness in the environment of actual service, the materials are required to exhibit electroconductivity sufficient for the purpose of preventing the members made thereof from being defiled by static electrification.
A review of the prior art from this point of view reveals the existence of metallic low thermal expansion materials represented by Invar and Super Invar, low thermal expansion glass, and various low thermal expansion ceramics such as cordierite, spodumene, and aluminum titanate as low thermal expansion materials.
Though Super Invar is characterized by exhibiting a relatively low thermal expansion coefficient of 1.3×10−7/K at ordinary room temperature and high electroconductivity, it is handicapped by possessing marked low specific rigidity of less than 20 GPa/g/cm3 as compared with ordinary ceramic materials. That is, the metallic low thermal expansion materials are extremely disadvantageous in terms of specific rigidity because they suffer from high specific gravity and relatively low Young's modulus as well.
Generally, ceramic materials are advantageous in terms of specific rigidity. As such a material, the low thermal expansion glass which has undergone a treatment for partial crystallization is disclosed in JP-A-50-132017 and in the catalog of Schott Corp. introducing its product sold under the trademark designation of “Zerodur.” The ceramic material which has undergone a treatment of partial crystallization, owing to the coexistence therein of a crystalline part and a vitreous part having thermal expansion coefficients with different signs, realizes the low thermal expansion by counterbalancing the different thermal expansions of the two parts throughout the whole of the material. The low thermal expansion glass of this quality, however, is handicapped by lacking sufficient electroconductivity notwithstanding the thermal expansion coefficient thereof is practically nil at room temperature. The specific rigidity thereof is about 35 GPa/g/cm3, a magnitude of which, though surpassing that of Super Invar, can hardly be rated as satisfactory.
The so-called low thermal expansion ceramics such as cordierite, spodumene, and aluminum titanate exhibits neither necessarily high specific rigidity nor fully sufficient electroconductivity.
In the technical field different from that of the present invention, the technique concerning the electroconductive low thermal expansion material that is aimed at providing a heater material enjoying exalted thermal shock properties has been in existence. The technique, however, is at a disadvantage in failing to impart fully satisfactorily low thermal expansion to the produced material.
JP-B-53-47514 and JP-B-60-37561, for example, disclose electroconductive low thermal expansion ceramics having an electroconductive substance dispersed in substances of a negative thermal expansion coefficient or a very small positive thermal expansion coefficient. The inventions of these publications are directed toward accomplishing low thermal expansion by dispersing a compound possessing a positive thermal expansion coefficient in the matrix formed of a compound possessing a negative or very small positive thermal expansion coefficient and thereby counterbalancing or lowering the mutual thermal expansions throughout the whole of the material. In this respect, these inventions have utilized the same technique disclosed in JP-A-50-132017 mentioned above. The inventions of JP-B-53-47514 and JP-B-60-37561, however, are directed toward a technique which is characterized by using as the compound to be dispersed in the matric phase a uniphase electroconductive substance and allowing at least part of this substance to continue and form a network throughout the whole of the material and thereby securing electroconductivity of the entire material. These ceramics, however, have failed, as demonstrated in examples cited in JP-B-53-47514, to realize satisfactorily low thermal expansivity because they require to disperse a large quantity of an electroconductive phase and, therefore, the absolute values of their thermal expansion coefficients are at least 4.2×10−7/K, a markedly large magnitude as compared with the thermal expansion coefficient of Super Invar.
Generally, since an electroconductive substance has a large thermal expansion coefficient, a matrix containing an electroconductive phase in a large ratio cannot realize a low thermal expansion property. If this matrix conversely has a small electroconductive phase content, it will be unable to acquire satisfactory electroconductivity. JP-B-53-47514, for example, cites an example which demonstrates the dependency of specific resistance and thermal expansion coefficient on the quantity of the electroconductive material incorporated in the material. The results of this example deny that such satisfactory electroconductivity and low thermal expansion properties are at present simultaneously fulfilled.
The technique which, as disclosed in JP-B-53-47514, secures electroconductivity of the whole material by incorporating an electroconductive phase in a matric phase which is an insulating material, causing at least part of the electroconductive phase to be dispersed in a continued state, and thereby forming a network of the electroconductive phase throughout the whole of the material. Gurland's report (Gurland, J., 1966, Trans. Metals Soc. AIEM, vol. 236, 642) is available for referential use. According to this report, as demonstrated by an experiment in a Bakelite-silver particles system, when the quantity of the electroconductive substance is not less than about 30% by volume, the electroconductive substance dispersed in the insulating material attains thorough mutual contact enough to realize electroconductivity of the whole material. It is extremely difficult for the reason stated above to realize a satisfactorily low thermal expansion property with the quantity of the electroconductive substance set in the neighborhood of this volumetric ratio. No case of succeeding in realizing this property has ever been reported to literature.
Even the invention of JP-B-60-37561, which utilizes a technique similar to the technique of JP-B-53-47514, requires a substance of low thermal expansion coefficient to incorporate therein not less than 25% by volume of an electroconductive phase and, consequently, fails to realize low thermal expansion in addition to securing satisfactory electroconductivity.
The addition of carbon black to a material of low thermal expansion is possibly used, as mentioned in JP-A-11-343168, as a means for blackening the material with the object of imparting a sunproofing property. Though carbon black exhibits electroconductivity, the sole addition of carbon in the quantity specified in the specification to cordierite does not result in acquiring such satisfactory electroconductivity as mentioned above.
The task which this invention aims to fulfill, therefore, resides in solving the problems confronting the prior art and providing a material of low thermal expansion which exhibits high specific rigidity and satisfactory electroconductivity as well with a view to realizing such precision machine members as demand a high degree of cleanliness and enjoy light weight and high dimensional accuracy.